The buckling is further back from the main cracks in the hull, and the ship is still holding in one piece for now.

An observation flight this morning had to turn back at Motiti Island due to the poor weather.

"The situation is looking increasingly precarious and we are preparing for the worst," Mr Anderson said.

"The worst of the weather is due to hit around midnight tonight. We are continuing to closely monitor the situation."

About 358 tonnes of oil remain in the submerged starboard number five tank, which salvors were building a water-tight corridor to until they had to stop work yesterday.

MetService says the bad conditions were not expected to ease until wind directions change tomorrow.

Mr Anderson said the tug Go Canopus would attempt to tow the stern to shallower water if it detached from the bow. However, the tug would only remain connected as long as it was safe to do so.

"Rena held up well through the bad weather two weeks ago, but we may not be so fortunate this time," Mr Anderson said.

Specialist container recovery company Braemar Howells Ltd has been contracted to deal with containers that may be released from Rena, and has four tugs on standby in Tauranga, with a further two on-site monitoring Rena in case containers fall overboard.

"The team at Braemar is making every effort to ensure they are ready to corral and secure released containers.

"However, as always, safety is the priority and the poor weather means there will only be so much that can be done initially."

The ship spilled more than 300 tonnes of oil into the sea after its hull was pierced in stormy seas on October 11, making the grounding New Zealand's worst maritime environmental disaster.

"I think the Bay of Plenty region should prepare for some oil to appear on the beaches in the next few days," Associate Transport Minister Nathan Guy said yesterday.

"Of course this may not happen, but we are prepared for a considerable bout of bad weather."

Maritime New Zealand salvage unit manager Kenny Crawford said salvors had sealed up the areas they had been working in as a precaution against the rough weather.

"This should help limit further release of oil in the worst-case scenario of the ship breaking up."

More containers were expected to be lost and salvors were fitting tracking transponders to accessible dangerous goods containers and others most likely to fall overboard.

Officials have also warned of more oil leaking from the Rena's damaged duct keel and other pockets in the ship.

But the rough weather could help to aid the natural dispersion of some of the oil, said national on-scene commander Mick Courtnell.

Salvage master Captain Drew Shannon could not guarantee the ship would hold together through the storm.

"We will just continue to monitor the condition of the ship the best we can."

The National Oiled Wildlife Response Centre was meanwhile continuing to expand, with more than 400 birds and animals now being cared for at the Mt Maunganui facility.